FIELD OF THE INVENTION
The invention relates to a gas and steam turbine plant with a waste-heat steam generator which is disposed downstream of a gas turbine on the exhaust-gas side and which has heating surfaces connected into a water-steam circuit of a steam turbine that has a high-pressure part, a medium-pressure part and a low-pressure part. The invention also relates to a plant operated according to the method.
In a gas and steam turbine plant, heat contained in expanded operating medium from the gas turbine is used to produce steam for the steam turbine. The transfer of heat is effected through the use of a number of heating surfaces, which are disposed in the form of tubes or tube bundles in a waste-heat steam generator located downstream of the gas turbine, on the exhaust side. Those surfaces, in turn, are connected into the water-steam circuit of the steam turbine. The water-steam circuit includes a plurality, e.g. two or three, pressure stages, each pressure stage having a preheater, an evaporator and a superheater as heating surfaces.
In order to achieve as high an efficiency of the plant as possible in the process of heat transfer, the configuration of the heating surfaces within the waste-heat steam generator is matched to the temperature profile of the exhaust gas of the gas turbine. In a three-pressure process with reheating, which is referred to as the three-pressure reheating process, a particularly high steam-turbine power and thus a particularly high overall efficiency of the plant is achieved for a given gas-turbine power. A gas and steam turbine plant operating in accordance with the three-pressure reheating process is known from European Patent 0 436 536 B1 and also from German Patent DE 44 34 526 C1, corresponding to allowed U.S. application Ser. No. 08/826,240, filed May 27, 1997. However, with those known plants as well, the overall efficiency in each case is limited to about 55%.